If you can't remember how to spell a word, you need to really look at it closely to find ways to help you to remember. Look at this word: 'separate'. Some people write 'seperate' or 'separite' - these are wrong. Just remember there's 'a rat' in it separate.
Try to look for words inside other words. Here's an exercise you can try: http://www.elgweb.net/wordword1.htm . Find the short words inside the long words. The letters are in the same order.
If you don't know the meaning of some of the long words, check them in a dictionary, then try http://www.elgweb.net/wordword2.htm to try to spell the long words.
Do you know more words that have other words in them? Please put them in the comments if so.